System for articular surface replacement

ABSTRACT

A system for replacing a portion of an articular surface including providing an implant site and installing an implant into the implant site. The implant site includes a first and a second excision site which at least partially intersect with one another. Each of the first and second excision sites are formed by providing a respective axis and excising a portion of the articular surface relative to the respective axes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional patentapplication Ser. No. 60/583,549, filed Jun. 28, 2004. This applicationis also a continuation-in-part of U.S. patent application Ser. No.10/308,718, filed Dec. 3, 2002. This Application is also acontinuation-in-part of U.S. patent application Ser. No. 10/994,453,filed Nov. 22, 2004, which claims the benefit of U.S. provisional patentapplication Ser. No. 60/523,810, filed Nov. 20, 2003, and which is acontinuation-in-part of U.S. patent application Ser. No. 10/308,718,filed Dec. 3, 2002. The entire disclosures of all of which applicationsare incorporated herein by reference.

FIELD

The present disclosure relates generally to the replacement of a portionof an articular surface.

BACKGROUND

Articular cartilage, found at the ends of articulating bone in the body,is typically composed of hyaline cartilage, which has many uniqueproperties that allow it to function effectively as a smooth andlubricious load-bearing surface. However, when injured, hyalinecartilage cells are not typically replaced by new hyaline cartilagecells. Healing is dependent upon the occurrence of bleeding from theunderlying bone and formation of scar or reparative cartilage calledfibrocartilage. While similar, fibrocartilage does not possess the sameunique aspects of native hyaline cartilage and tends to be far lessdurable.

Hyaline cartilage problems, particularly in knee and hip joints, aregenerally caused by disease such as occurs with rheumatoid arthritis orwear and tear (osteoarthritis). Hyaline cartilage problems may also bethe result of an injury, either acute (sudden) or recurrent and chronic(ongoing). Such cartilage disease or deterioration can compromise thearticular surface causing pain and further deterioration of jointfunction. As a result, various methods have been developed to treat andrepair damaged or destroyed articular cartilage.

For smaller defects, traditional options for this type of probleminclude non-operative therapies (e.g., oral medication or medication byinjection into the joint), or performing a surgical procedure calledabrasion arthroplasty or abrasion chondralplasty. The principle behindthis procedure is to attempt to stimulate natural healing. At the defectsite, the bone surface is abraded, removing approximately 1 mm. or lessusing a high-speed rotary burr or shaving device. This creates anexposed subchondral bone bed that will bleed and will initiate afibrocartilage healing response. Although this procedure has been widelyused over the past two decades and can provide good short term results,(1-3 years), the resulting fibrocartilage surface is seldom able tosupport long-term weight bearing, particularly in high-activitypatients, and is prone to wear.

Another procedure, referred to as the “microfracture” technique,incorporates similar concepts of creating exposed subchondral bone.During the procedure, the cartilage layer of the chondral defect isremoved. Several pathways or “microfractures” are created to thesubchondral bleeding bone bed by impacting a metal pick or surgical awlat a minimum number of locations within the lesion. By establishingbleeding in the lesion and by creating a pathway to the subchondralbone, a fibrocartilage healing response is initiated, forming areplacement surface. Results for this technique are generally similar toabrasion chondralplasty.

Another known option to treat damaged articular cartilage is a cartilagetransplant, referred to as a Mosaicplasty or osteoarticular transfersystem (OATS) technique. This technique involves using a series of dowelcutting instruments to harvest a plug of articular cartilage andsubchondral bone from a donor site, which can then be implanted into acore made into the defect site. By repeating this process, transferringa series of plugs, and by placing them in close proximity to oneanother, in mosaic-like fashion, a new grafted hyaline cartilage surfacecan be established. The result is a hyaline-like surface interposed witha fibrocartilage healing response between each graft.

Such an OATS procedure is technically difficult, as all grafts must betaken with the axis of the harvesting coring drill being keptperpendicular to the articular surface at the point of harvest. Also,all graft placement sites must be drilled with the axis of a similarcoring tool being kept perpendicular to the articular surface at thepoint of implantation. Further, all grafts must be placed so that thearticular surface portion of these cartilage and bone plugs is deliveredto the implantation site and seated at the same level as the surroundingarticular surface. If these plugs are not properly placed in relation tothe surrounding articular surface, the procedure can have a verydetrimental effect on the mating articular surface. If the plugs areplaced too far below the level of the surrounding articular surface, nobenefit from the procedure will be gained. Further, based on therequirement of perpendicularity on all harvesting and placement sites,the procedure requires many access and approach angles that typicallyrequire an open field surgical procedure. Finally, this procedurerequires a lengthy post-operative non-weight bearing course.

Transplantation of previously harvested hyaline cartilage cells from thesame patient has been utilized in recent years. After the cartilage isremoved or harvested, it is cultured in the lab to obtain an increase inthe number of cells. These cells are later injected back into the focaldefect site and retained by sewing a patch of periosteal tissue over thetop of the defect to contain the cells while they heal and mature. Thedisadvantages of this procedure are its enormous expense, technicalcomplexity, and the need for an open knee surgery. Further, thistechnique is still considered somewhat experimental and long-termresults are unknown. Some early studies have concluded that thisapproach offers no significant improvement in outcomes over traditionalabrasion and microfracture techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the present invention are set forth bydescription of embodiments consistent with the present invention, whichdescription should be considered in conjunction with the accompanyingdrawings wherein:

FIG. 1 a is an exploded top perspective view of an embodiment of a lowprofile cutting system;

FIG. 1 b is an exploded bottom perspective view of the cutting systemshown in FIG. 1 a,

FIG. 2 is a perspective view of the low profile cutting system shown inFIG. 1 a with the cutter assembled to the support member;

FIG. 3 depicts, in perspective view, one application of the cuttingsystem shown in FIG. 1 a;

FIG. 4 shows in side elevation one application of the cutting systemillustrated in FIG. 1 a;

FIG. 5 illustrates an embodiment of a drill guide that may be used witha cutter system of the present disclosure;

FIG. 6 is a cross-sectional view schematically illustrating the use of acutting system and drill guide according to the present disclosure toexcise an articular surface;

FIG. 7 depicts a drive element that may suitably be used for driving acutter according to FIG. 1 a;

FIG. 8 is an enlarged view of the head of the drive element shown inFIG. 7;

FIG. 9 is a cross-sectional view shown an articular surface including aregion excised using a cutting system according to the presentdisclosure;

FIG. 10 is a cross-sectional view schematically illustrating anotherembodiment of a cutting system and drill guide according to the presentdisclosure;

FIG. 11 is an enlarged view of the cutting system and drill guide ofFIG. 10 being used to excise a region of an articular surface;

FIG. 12 is a top perspective view of an embodiment of a first implant;

FIG. 13 is a bottom perspective view of the first implant shown in FIG.12;

FIG. 14 illustrates a cross-sectional view of a tibia including anarticular surface implant;

FIG. 15 schematically illustrates a knee joint including an embodimentof a uni-compartmental joint replacement;

FIG. 16 depicts an embodiment of a drill guide for preparing an implantsite on an articular surface;

FIG. 17 illustrates the embodiment of a drill guide shown in FIG. 15positioned on an articular surface;

FIG. 18 is a side elevation schematic drawing depicting an implantprovided by two overlapping implants;

FIG. 19 is an underside perspective schematic drawing of implant shownin FIG. 18;

FIG. 20 is an side elevation view of the implant shown in FIG. 18;

FIG. 21 is a perspective view of an embodiment of a low profile cuttingsystem consistent with the present disclosure;

FIG. 22 is an enlarged cross-sectional view of the cutting systemillustrated in FIG. 21;

FIG. 23 is an enlarged cross-sectional view of the cutter support of thecutting system illustrated in FIG. 21; and

FIG. 24 illustrates the cutting system show in FIG. 21 being employed inconjunction with a drill guide.

DESCRIPTION

The present disclosure relates to a system for replacing a portion of anarticular surface. While the methods and instruments described hereinare particularly directed at the replacement of a portion of anarticular surface of a femoral condyle and/or a tibial condyle, thesystem herein may suitable by employed for replacing a portion of otherarticular surfaces. Additionally, while the disclosure is relates toreplacing a portion of each of two cooperating articular surfaces, thesystem herein may be used to replace only a portion of one articularsurface of a joint, without necessitating the replacement of acooperating articular surface of the joint.

Referring to FIGS. 1 through 9, a system for replacing a portion of anarticular surface is disclosed. Generally, the disclosed system includesidentifying a portion of an articular surface to be replaced, forexample a damaged portion of the articular surface, or a portionincluding a lesion or defect, excising the identified portion of anarticular surface to create an implant site and installing an implant inthe implant site. The portion of the articular surface to be replacedherein may be identified arthroscopically, for example using eitherdiagnostic or surgical arthroscopy. According to an embodiment, aportion of an articular surface of a tibia may be replaced. In such anembodiment, the knee joint may be partially opened by pulling the femuraway from the tibia, to increase the clearance between the articularsurface of the tibia being replaced and the cooperating articularsurface of the femur. The increased clearance between the correspondingarticular surfaces of the tibia and the femur may facilitate access tothe identified portion of the articular surface of the tibia to enableexcising the identified portion of the articular surface to create animplant site. According to another embodiment, at least a portion of thecorresponding articular surface of the femur may be resected prior toreplacing a portion of the articular surface of the tibia. Resection ofat least a portion of the articular surface of the femur may provide anincrease in the clearance between the articular surface of the tibia andthe remaining corresponding portion of the femur. The resultingincreased clearance may facilitate access to the articular surface ofthe tibia. An exemplary method for resecting at least a portion of thearticular surface of the femur is discussed in detail infra.

Turning first to FIGS. 1 a, 1 b, and 2, an embodiment of a low profilecutting system 10 is shown. The low profile cutting system 10 maygenerally include a support member, generally indicated at 12, and acutter, generally indicated at 14. The support member 12 may include ashaft 16 including a cutter support 18 on the distal end thereof. Thecutter support 18 may be provided as a ring or hoop and define acircular opening 20 sized to receive the cutter 14 therein, as shown inFIG. 2. The cutter support 18 may also be provided in configurationsother than a ring or a hoop. For example, the cutter support 18 may havea square, rectangular, polygonal, oval, etc. shape in plan view,defining an opening for receiving the cutter 14.

The shaft 16 of the support member 12 may include a lumen, not shown,providing access via the shaft 16 to the opening 20 of the cuttersupport 18. The lumen defined by the shaft 16 may provide suction,supply a fluid, such as for flushing the region of the cutter support18, etc. Alternatively, an inner shaft (not shown) may be translatablydisposed at least partially within the lumen. The inner shaft may be atleast partially extensible into the opening 20 defined in the support18. The inner shaft may be urged into or towards the opening 20, forexample, and act against an edge of a member disposed in the opening 20.The pressure of the inner shaft bearing against a member disposed in theopening 20 may aid in maintaining the member in the opening, forexample, be creating a frictional interaction between the member and thesupport 18 and/or the inner shaft.

The shaft 16 may also include features, such as collar or ring 22, aswell as other features and/or indicia along the length and/orcircumference of the shaft 16 to facilitate manipulating the supportmember and/or for providing reference for use in conjunction withcooperating instruments. For example, the features or indicia may beused to align and/or position other instruments in a desiredrelationship with the support member and/or the cutter support 18 oropening defined therein.

The support member 12 may also include a shoe 24 that may be radiallymoveable into the opening 20. In one embodiment, the shoe 24 may form aportion of the circumference of the opening 20 in a retracted position,and extend into the opening 20 in a second position. The shoe 24 may bebiased to a position extending into the opening 20, for example by aspring or a translatable shaft disposed in the lumen of the shaft 16.Accordingly, when the cutter 14 is disposed within the opening 20 theshoe 24 may be in a retracted position, i.e., at or withdrawn from thecircumference of the opening 20, and when the cutter 14 is removed fromthe opening 20 the shoe 24 may be in an extended position, i.e.,extending at least partially into the opening 20. In one embodiment, itis contemplated that the shoe 24 may have a height less that the heightof the cutter support 18. Accordingly, the shoe 24 may move to anextended position when the cutter 14 is still partially received in theopening 20.

The cutter 14 may be formed having a generally circular cross-sectionsized to be rotatably received within the opening 20 of the supportmember 12. The diameter of the cutter 14 may be sized to allow thecutter 14 to be guided by the cutter support 18 with a minimal of playor run-out. The cutter 14 may be configured for cutting or boring in anaxial direction.

The cutter 14 may include a cutting face 26. The cutting face 26 mayengage an articular surface in which an implant site is to be formed.The cutting face 26 may include features suitable for excising thearticular cartilage and/or underlying bone to form an implant site.According to one embodiment, cutting face 26 of the cutter 14 mayinclude a taper, for example providing the cutting face 26 having ashallow conical surface. The cutting face 26 may also include acentering point 27. Consistent with the illustrated embodiment, thecentering point 27 may be a conical protrusion, or other protrusionproviding a point. The centering point 27 may establish, and maintain,the cutter 14 along an axis extending through the center of the cutter.Also as shown in the illustrated embodiment, the cutter 14 may includecut-outs, or flutes, 28, 30. The flutes 28, 30 may provide leadingcutting edges, e.g. 32 in the illustrated embodiment, that are angledrelative to the axis of the cutter 14. According to one embodiment, theflutes 28, 30 may have a helical configuration about the axis of thecutter 14. The illustrated embodiment of the cutter 14 includes twoflutes 28, 30. However, alternative embodiments may be provided with amore or fewer flutes consistent with the present disclosure.

According to the above-described embodiment, the cutter 14 may beprovided resembling a section of a twist drill bit. As such, when theillustrated cutter 14 is rotated in a clockwise direction the leadingcutting edges 32 may cut into a surface or body to be excised, in asimilar manner to a drill bit. The flutes 28, 30 may allow debrisproduces during excision to be evacuated from the implant site beingformed, e.g. to the top of the cutter 14. Debris removed from theimplant site being formed may be collected, for example using a suctiondevice associated with, or positioned adjacent to the cutter support 18.Allowing debris to be evacuated from the implant site being formed mayreduce compacting of debris at the sides of the cutter 14, binding ofthe cutter 14, and/or facilitate faster and more efficient excision ofthe articular surface and/or underlying bone.

According to another embodiment, the cutting face 26 of the cutter mayinclude one or more cutting blades. The cutting blades may extend fromthe cutting face 26 either axially or at an angle to the axis of thecutter 14. Similarly, the cutting blades may be arranged extendingradially or at an angle. According to various embodiments, the cuttingblades may be integrally formed with the cutter, may be separatecomponents fixed to the cutter, or may be replaceable componentsremovably attached to the cutter. Similar to the illustrated embodiment,a cutter including cutting blades extending from the cutting face mayinclude flutes to facilitate the removal of debris from an excisionsite.

The cutter 14 may also include a top face 34. The top face 34 mayinclude an upstanding rib 36 around the circumference of the cutter 14.Effectively, the upstanding rib 36 may provide the top face 34 having arecessed configuration. The top face 34 may also be provided as anon-recessed feature, whereby the upstanding rib 36 may be eliminated.

Consistent with the present disclosure, the top face 34 may include adrive socket 38. The drive socket 38 may be configured to receive acooperating drive element to allow the transmission of torque from thedrive element to the cutter 14. As in the illustrate embodiment, thedrive socket 38 may be configured as a hexagonal socket, which mayreceive a hexagonal end of a drive element to allow torque to betransmitted to the cutter 14 from the drive element. The socket 38 mayinclude a chamfered upper edge 40 to facilitate insertion and engagementof a cooperating drive element. Various other socket configurations maysuitably be employed to allow transmission of torque from a driveelement to the cutter 14. For example, the socket 38 may include asplined socket, a flatted circular socket, a polygonal socket, etc.Additionally, it is contemplated herein that rather than a socket, thecutter may include a protrusion, such as a hexagonal protrusion, etc.,that may be engaged by a socket on a drive element. Various othercombinations of features may also suitably be used for transmittingtorque between a drive element and the cutter.

Turning to FIGS. 3 and 4, one application of a cutting system 10according to the present disclosure is shown. Consistent with theillustrated application, the cutting system may be used to provide animplant site between two cooperating articular surfaces, for examplebetween an articular surface of a femur 100 and an articular surface ofa tibia 102. As shown, the shaft 12 may be manipulated to position thecutter support 18, including the cutter 14 received therein, within thejoint between the cooperating articular surfaces of the femur 100 andtibia 102. According to one embodiment, the joint may be slightly openedby withdrawing the tibia 102 away from the femur 100 to at leastslightly increase the clearance between the articular surfaces of thefemur 100 and tibia 102.

Consistent with the system herein, at least a portion of the cooperatingarticular surface, e.g., an articular surface of the femur, may beresected prior to creating an implant site using the cutting system 10herein. Resecting at least a portion of a cooperating articular surfacemay increase the clearance between the articular surfaces, therebyincreasing access to the articular surface to be excised by the cuttingsystem 10.

The shaft 16 may be manipulated to locate the opening 20 of the cuttersupport 18, and thereby locate the cutter 14, in a desired position onthe articular surface. For example, the cutter 14 may be positionedgenerally centered over, or at least partially encompassing a defect inand articular surface. The defect may include, for example, a region ofdamaged articular cartilage, a lesion, etc. Furthermore, the cutter 14may be positioned in a location of a desired auxiliary implant, such asan implant capable of interacting with an implant on a cooperatingarticular surface, for example to provide smoother operation or improvedwear characteristics relative to native cartilage. Consistent with oneembodiment, the shaft 12 may be manipulated to push the meniscus 104aside with the cutter support 18. Accordingly it may be possible todirectly expose an articular surface to the cutter 14. By employing sucha technique, it may be possible to provide an implant on an articularsurface of a knee without damaging the meniscus. By preserving themeniscus in this manner, it may be possible to provide a joint includingarticular surface implants having a smoother operation, better shockabsorbing characteristics, better wear characteristics, and/or a morenatural feel as compared to a joint including articular surface implantsand having the meniscus removed. Preserving the meniscus is not,however, essential within the scope of the present disclosure.

Referring next to FIG. 5 a drill guide 200 that may be used incombination with the cutting system 10 is shown. The drill guide 200 mayinclude a first alignment feature 202 that may be removably coupled tothe cutting system 10 in a desired relationship. For example, the firstalignment feature 202 may include a bore or opening capable of receivingthe shaft 12 in a manner to align the first alignment feature 202axially with the shaft 12. Consistent with one embodiment, the shaft 12may be received through a bore in the first alignment feature 202 topermit the collar or ring 22 of the shaft 12 to bear against a portionof the first alignment feature 202, thereby locating the first alignmentfeature 202 along the axis of the shaft 12. The shaft 12 and/or thefirst alignment feature 202 may also include indicia and/or cooperatingfeature to achieve a desired radial alignment between the shaft 12 andthe first alignment feature 202. Consistent with the present disclosure,the first alignment feature 202 and/or the shaft 12 may be capable ofachieving a variety of desired relationships with respect to oneanother, e.g. by using sets of indicia or cooperating features. Thefirst alignment feature 202 and/or the cutting system 10 may include avariety of elements, such as screws, snap-fits, etc. for removablycoupling the first alignment feature 202 to the cutting system 10.

The drill guide 200 may also include a second alignment feature 204coupled in a desired relationship to the first alignment feature 202 bya support structure 206. The support structure 206 may include an armextending between the first and second alignment features 202, 204. Inthe illustrated embodiment the support structure 206 is provided as acontinuous arcuate member. Various other embodiments of the supportstructure 206 may suitably be employed for providing a desiredrelationship between the first and second alignment features 202, 204.For example, the support structure 206 may include one or more straightsections and/or additional features.

Furthermore, consistent with one embodiment, the support structure 206may be adjustable and/or capable of providing a plurality ofrelationships between the first and second alignment features 202, 204.The support structure 206 may be adjustably coupled to one or both ofthe first and second alignment features 202, 204. The support structure206 may also, or alternatively, include adjustable portion along theextent of the support structure 206 capable of adjusting therelationship between the first and second alignment features 202, 204.One or more adjustable features of the drill guide 200 may includeindicia and/or cooperating features to facilitate positioning the firstand second alignment features 202 in desired and/or predeterminedrelationships relative to one another.

According to one embodiment the second alignment feature 204 may includea bore 208 extending therethrough. The drill guide 200 may be providedto position the bore 208 in predetermined relationship relative to thecutting system 10. According to one such embodiment the drill guide 200may configured to provide a desired relationship between the bore 208and the cutter 14 received in the cutter support 18 of the cuttingsystem 10. For example, the drill guide 200 may be configured to alignthe bore 208 to have an axis that intersects the socket 38 of the cutter14 at a desired angular relationship.

In one embodiment according to the present disclosure, the drill guide200 may be coupled to the cutting system 10 to orient the bore 208 ofthe second alignment feature 204 along an axis that may intersect thesocket 38 of the cutter 14. With the drill guide 200 provided in such anarrangement, a drill bit (not shown) may be disposed extending throughthe bore 208 of the second alignment feature 204. Using the drill bit, ahole may be drilled through a portion of the femur 100, including thearticular surface thereof, to expose the socket 38 of the cutter 14.

Turning now to FIGS. 6 through 9, after a hole has been drilled toexpose the socket 38 of the cutter 14, the cutting system 10 and drillguide 200 may be used to excise a portion of the articular surface ofthe tibia 102 and/or of the underlying bone. Once the hole, generallyindicated at 300, has been drilled through the femur 100 extending tothe articular surface thereof to expose the socket 38 of the cutter 14,the drill bit (not shown) may be withdrawn from the hole 300 and fromthe second alignment feature 204. A drive element 302 may then beinserted through the second guide feature 204 and through the hole 300and engaged with the cutter 14.

Referring to FIGS. 7 and 8, an embodiment of a drive element 302 isshown. As depicted, the drive element 302 may include a shaft 304portion and a head 306 portion. The shaft portion 304 may include agenerally cylindrical shaft. A proximal end of the shaft 304 may beadapted to be engaged by a driver (not shown), such as a drill. While acylindrical shaft may suitably be engaged by a conventional drill chuck,the proximal end of the shaft 304 may also include features specificallyadapted for transmitting torque from the driver to the shaft 304. Forexample, the proximal end of the shaft may include flatted surfaces orthe like to facilitating gripping of the shaft 304 as well astransmitting torque to the shaft 304. The proximal end of the shaft 304may also include features capable of engaging with specific and/orproprietary drivers.

The head portion 306 of the drive element may include a feature capableof being received in and capable of engaging the socket 38 of the cutter14. In the illustrated embodiment, the head 306 may include a featurehaving a hexagonal cross-section. The hexagonal cross-section of thehead 306 may be sized to be received in the socket 38 and to transmittorque from the drive element 302 to the cutter 14. The head portion 306may have different configurations, such as cross-sectional geometry, tosuit different specific sockets 38. Furthermore, the head 306 of thedrive element 302 may be provided including a socket in a case in whichthe cutter 14 is provided having a protrusion rather than a socket.

According to one embodiment, the head portion 306 may be configured as aball-drive feature, as shown in the illustrated embodiment. Theball-drive head portion may allow the drive element 302 to transmittorque to the cutter in situations in which the drive element 302 is notcoaxially aligned with the socket 38 and/or the cutter 14. As shown, aball-drive head may include a generally rounded or spherical featurewhen viewed in profile. The rounded aspect of the ball-drive headportion 306 may allow the drive element 302 to pivot within the socket38 generally about a center of the ball of the head portion 306. In sucha manner, when torque is transmitted to the cutter 14 via the driveelement, the ball-drive head portion 306 in combination the socket 38may perform in a manner similar to a universal, or flex, joint betweenthe cutter 14 and the shaft portion 304 of the drive element 302.Accordingly, utilizing a ball-drive head portion 306, while notnecessary, may increase the acceptable tolerance relative to thealignment of the drive element 302 and the socket 38 and/or cutter 14.

In the illustrated embodiment, the ball-drive head portion 306 is shownhaving a diameter that is larger than the diameter of the shaft portion304. This configuration may allow the drive element 302 to transmittorque to the cutter 14 even when the drive element 302 is out ofalignment with the axis of the cutter 14. In some cases the driveelement 302 may be able to transmit torque to the cutter 14 up to anangle at which the shaft portion 302 begins to bind or drag against anupper edge of the socket 38, e.g., the chamfered upper edge 40.

In another embodiment, the shaft portion 304 of the drive element mayhave a diameter that is equal to, or greater than, the head portion 306of the drive element 302. According to such an embodiment, it may stillbe possible to provide the drive element 302 including a ball-drive headportion 306, thereby allowing a margin or misalignment between the driveelement 302 and the cutter 14. When the shaft portion 304 is providedhaving a diameter equal to or greater than the head portion 306, thehead portion 306 may be provided having a ball-drive configuration byproviding a neck disposed between the shaft portion 304 and the headportion 306. The neck may include a region having a smaller diameterthan the shaft portion 304 or the head portion 306. The neck may includea transition region extending between the region having a smallerdiameter and each of the shaft portion 304 and the head portion 306. Theneck may allow some misalignment between the drive element 302 and thecutter 14 by allowing the ball-drive head portion 306 to rotate in thesocket 38 without the shaft portion 304 of the drive element 302 bindingagainst and edge of the socket 38.

As shown in FIG. 9, the overall system illustrated in FIG. 6 may be usedto excise a portion of an articular surface, of a tibia in the case ofthe illustrated embodiment, as well as a portion of the bone underlyingthe articular cartilage, to provide an implant site 400. The implantsite 400 may be formed by driving the cutter 14 with a driver, such as apower drill, via the drive element 302. The cutter 14 may be drivenuntil the implant site 400 has been excised to a desired depth.According to one embodiment, the depth of the implant site may be gaugedusing the shoe 24. The shoe 24 may be biased toward a position extendinginto the opening 20 of the cutter support 18. As the cutter 14 is drivento a depth below a bottom edge of the shoe 24 the shoe 24 may move intoa position extending into the opening 20 as a result of the spring bias.The movement of the shoe 24 into an extended position may be visuallyperceptible, or may trigger an alert condition. The alert condition mayinclude an audible or visual indicator that may result from anelectrical signal generated by the movement of the shoe 24 into theextended position, or may be caused by a mechanical actuation as aresult of the movement of the shoe 24 into the extended position. Othermethods and apparatus for determining the depth of an excised implantsite 400 may also be employed consistent with the present disclosure.

After an implant site 400 having a desired depth has been excised, thecutter 14 may be extracted. In an embodiment the cutter 14 may beextracted by withdrawing the cutter 14 back into the opening 20 of thecutter support 18. The cutter 14 may then be withdrawn from the jointsite by extracting the cutting system 10, including the cutter 14,cutter support 18, and shaft 12, as a single unit. The cutter 14 may bewithdrawn back into the opening 20 of the cutter support 18 using, forexample, a pick that may be manipulated extending through the hole 300previously created extending through at least a portion of the femur.The pick may pull the cutter 14 upwardly from the implant site 400 andinto the cutter support. According to one embodiment, the drive element302 may be employed to withdrawn the cutter 14 into the cutter support18. In such an embodiment, the drive element 302 may be provided with ameans for releasably retaining the cutter 14. The drive element 302 mayinclude, for example, an extendable feature from the head portion 306capable of releasably retaining the cutter 14. In an embodiment of thecutting system 10 including a biased shoe 24, the shoe 24 may first bewithdrawn into a retracted position before the cutter 14 is withdrawninto the cutter support 18.

According to another embodiment, the cutter support 18 may first bewithdrawn from the joint site. The cutter 14 may then be extracted fromthe implant site 400. In one such embodiment, the cutter 14 may beextracted using, for example, a pick or other retraction deviceoperating between the articular surfaces of the femur 100 and tibia 102.Additionally, or alternatively, a retraction device operating throughthe hole 300 previously created extending through at least a portion ofthe femur may be used to extract the cutter 14 from the implant site400, and from the joint site.

Referring to FIGS. 10 and 11, another embodiment of a system 410 thatmay be employed to excise at least a portion of an articular surface isshown. As with the previous embodiment, the system 410 may generallyinclude a cutting system 10 b and a drill guide 200 b. The cuttingsystem 10 b may generally include a support member 12 b and a cutter 14b. The support member may include a shaft 16 b extending from a cuttersupport 18 b that includes an opening for receiving a cutter 14 b. Thesystem 410 may also include a drill guide 200 b that may be used incombination with the cutting system 10 b to excise a portion of anarticular surface of a joint. The drill guide 200 b may generallyinclude a first alignment feature that may be removably coupled to theshaft 16 b of the cutting system 10 b in a desired angular and/orrotational relationship. For example, in the illustrated embodiment, thefirst alignment feature 202 b includes a bore adapted to receive theshaft 16 b of the cutting system 10 b. The bore and/or the shaft 16 bmay include cooperating features and/or indicia to facilitate achievinga desired alignment. Such cooperating features and/or indicia may allowvariable desired alignments to be achieved.

The drill guide 200 b may also include a second alignment feature 204 bcoupled in a desired relationship to the first alignment feature 202 bby a support structure 206 b extending between the first alignmentfeature 202 b and the second alignment feature 204 b. As in theillustrated embodiment, the support structure 206 b may be configured asa continuous arc formed from a single piece. According to alternativeembodiments, the support structure 206 may include adjustment featuresthat enable the relationship between the first alignment feature 202 band the second alignment feature 204 b to be varied. The adjustmentfeatures may include indicia or cooperating features that enable thefirst alignment feature 202 b and second alignment feature 204 b to bepositioned in a desired predetermined relationship to one another.Furthermore, the support structure 206 b may be formed from generallystraight components and/or components having both straight and arcuateregions.

Similar to the previous embodiment, the second alignment feature 204 bmay include a bore extending through the second alignment feature 204 b.The bore 208 b may be adapted to receive a drill bit (not shown) orsimilar tool therethrough. The bore 208 b of the alignment feature maybe provided to position the drill bit in a desired orientation relativeto the cutting system 10 b. According to one embodiment, the drill guide200 b may be configured to position a drill bit to intersect the cuttersupport 18 b in a central region and aligned generally normal to thecutter 14 b. As with the previous embodiments, the bore 208 b of thesecond alignment feature 204 b may also be adapted to rotatably receivea drive element therethrough. In use, the drill guide 200 b may becoupled to the cutter system 10 b such that the second alignment feature204 b may be positioned to intersect a central region of the cuttersupport 18 b along an axis that is generally normal to the cutter 14 b.

With further reference to the enlarged view of FIG. 11, the cutter 14 bmay include a generally centrally located opening 38 b extending intothe cutter 14 b from the bottom thereof. The opening may define a socket38 b, and as such may be have a splined, keyed, polygonal, etc.configuration allowing torque to be transmitted to the cutter 14 b.According to one embodiment, the socket 38 b may extend all of the waythrough the cutter 14 b. As shown in FIGS. 10 and 11, the cutting system10 b may be positioned within the joint so that the cutter support 18 b,and the cutter 14 b disposed therein, is generally centered about adesired excision site. The drill guide 200 b may be coupled to thecutting system 10 b and oriented so that the second alignment feature204 b is generally aligned along an axis that intersects the center ofthe cutter 14 b and is generally oriented normal to the cutter 14 b. Adrill bit may be positioned extending through the second alignmentfeature 204 b, and a hole 300 b may be drilled through the tibia 102 b,in the illustrated embodiment. The hole 300 b may be drilled extendingthrough the tibia 102 b up to an articular surface thereof. The hole 300b may extend through the articular surface of the tibia 102 b andprovide access to the socket 38 b in the bottom side of the cutter 14 bfacing the articular surface of the tibia 102 b.

With the hole 300 b providing access through the tibia 102 b to thesocket 38 b of the cutter 14 b, a drive element including a drive shaft304 b and a drive head 306 b may be inserted through the secondalignment feature 204 b of the drill guide 200 and through the hole 300b in the tibia 102 b. A proximal end of the drive shaft 304 b may beadapted to be engaged by a driver (not shown), such as a drill. Thedrive head 306 b may be adapted to be received in the socket 38 b of thecutter 14 b. The drive head 306 b may also be adapted for transmittingtorque through from the drive shaft 304 b to the cutter 14 b. Forexample, the drive head 306 b and socket 38 b may each have a polygonalprofile, such as a hexagonal profile. Alternatively, the ball drive 306b and the socket 38 b may include features such as flats, splines, keys,etc. to facilitate the transmission of torque from drive shaft 304 b tothe cutter 14 b.

In addition to the torque transmitting features of the drive head 306 band the socket 38 b, the drive head 306 b and the socket 38 b mayinclude cooperating locking features. The locking features herein mayoperate to selectively prevent axial extraction of the drive head 306 bfrom the socket 38 b. Consistent with the illustrated embodiment, thelocking feature may include an indentation 308 in the drive head 306 b.The indentation 308 may be configured as a recess or opening in theouter circumference of the drive head 306 b. According to alternativeembodiments, the indentation 308 may include, for example, a groovearound the exterior of the drive head 306 b. The cutter 14 b may includea cooperating detent (not shown) for engaging the indentation in thedrive head 306 b. According to an embodiment herein, the cutter 14 b mayinclude a radial opening 42. The opening 42 may house a plug (not shown)such as a cylindrical member, a ball, etc. that may be biased, e.g., bya spring, toward the interior of the socket 38 b. Consistent with suchan arrangement, when the drive head 306 b is inserted into the socket 38b the plug may be radially displaced toward the circumference of thecutter 14 b. Once the drive head 306 b has been inserted into the socked38 b such that the indentation 308 is generally aligned with the opening42, the biased plug may move toward the center of the cutter 14 b and aportion of the plug may engage the indentation 308 in the drive head 306b. According to such an embodiment, the drive head 306 b may bereleasably engaged with the cutter 14 b and resist axial extraction ofthe drive head 306 from the socket 38 b until an extracting force isapplied to the drive head 306 b sufficient to overcome the bias on theplug and force the plug toward the circumference of the cutter 14 b.Various other locking features and mechanism may also be employed forreleasably engaging the drive head 306 b in the socket 38 b.

The drive shaft 304 b and/or the drive head 306 b may also includefeatures that permit engagement between the drive head 306 b and thesocket 38 b even in the event of a slight to moderate misalignmentbetween drive shaft 304 b and the cutter 14 b. As in previousembodiments, the drive head 306 b may include a ball driveconfiguration. As discussed above, a drive ball configuration of thedrive head 306 b may allow the connection between the drive head 306 band the socket 38 b to operate in a manner similar to a universal joint.Similarly, the drive shaft 304 b may include a flexible section, forexample, disposed adjacent the drive head 306 b. Other similar featuresmay also be employed.

Consistent with the foregoing, once the drive head 306 b is received inthe socket 38 b and axially locked in the socket 38, an implant site maybe excised in an articular surface of the tibia 102 b using the cutter14 b. A driver, for example a power drill, may be used to rotationallydrive the cutter 14 b via the drive shaft 304 b and drive head 306 bengaged in the socket 38 b. As the cutter 14 b is being rotationallydriven by the driver, an axially withdrawing force may be applied to thedrive shaft 304 b urging the cutter 14 b into the articular surfacebeing excised, e.g. an articular surface of a tibia in the illustratedembodiment. The locking feature coupling the drive head 306 b to thesocket 38 b may prevent the drive head 306 b from being extracted fromthe socket 38 b, and therefore allow the axially withdrawing forceapplied to the drive shaft 304 b to be transmitted to the cutter 14 b,and thereby urge the cutter into the articular surface. From a generalstandpoint, the cutter 14 b is being rotated and pulled into thearticular surface.

The cutter 14 b may be driven until the articular surface has beenexcised to a desired depth. The depth of the excised site may bemonitored and/or checked using a variety of techniques. On suchtechnique is simple observation at the articular surface, e.g., viewingthrough the cutter support 18 b. A biased shoe may also be used tomonitor the depth of the excision. That is, the cutting system 10 b mayinclude a shoe biased toward a position extending inwardly into thecutter support 18 b, wherein movement of the shoe to an extendedposition may be impeded by the cutter 14 b. When the excision depth issufficient that the cutter 14 b is below the shoe, the shoe may moveinwardly to an extended position. The movement and/or the position ofthe show may be visibly perceptible, and/or may be coupled to anindication system providing visible and/or audible indication that theshoe has moved to an extended position. According to still anotherembodiment, the drive shaft 304 b and/or the second alignment feature204 b may include indicia representative of the position of the cutter14 b when the drive head 306 b is received in the socket 38 b of thecutter 14 b. As the articular surface is excised and the cutter movesinto the articular surface the drive shaft 304 b may be extracted fromthe second alrgnment feature 204 b a corresponding distance. The depthof the excised site may, therefore, be determined by reference to theamount the drive shaft 304 b is withdrawn.

Referring to FIGS. 21 through 24, an embodiment of a cutting system 10 cmay be provided in which the cutter support 18 c may be provided havinga low profile, that is, the height of the cutter support 18 c along theaxis of the opening 20 c may be small enough to allow the cutter support18 c to be inserted in between the cooperating articular surfaces of thefemur 100 and tibia 102. According to the illustrated embodiment, thecutter support 18 c may be distally tapered such that the proximal endof the cutter support 18 c has a greater height than the distal end ofthe cutter support 18 c. As best seen in FIG. 23, the cutter 14 c mayalso be provided having a low profile to be received in the low profilecutter support 18 c. As such, the top surface 34 c of the cutter 14 cmay be radially tapered such that the center of the cutter 14 c in theregion of the socket 38 c may have a height that is greater than theheight of the cutter 14 c at the outer edge thereof.

With reference to FIGS. 22 and 23, the cutting system 10 c may includean internally threaded knob 50 rotatably disposed on the support member12 c. The internal threads 52 of the knob 50 may engage a threadedportion of a translatable shaft 54 disposed within a lumen of the shaft16 c. Rotation of the knob 50 may act to advance or withdraw the shaft54 relative to the cutter support 18 c. The translatable shaft 54 may beeither directly or indirectly coupled to the shoe 24 c. Advancing thetranslatable shaft 54 may cause the shoe 24 c to bear against thecircumference of the cutter 14 c. According to one embodiment, when theshoe 24 c may bear against the circumference of the cutter 14 c, thecutter may translate slightly and engage a lip 56 on the cutter support18 c, thereby securely retaining the cutter 14 c in the cutter support18 c. The cutter 14 c may be released by rotating the knob 50 towithdraw the shoe 24 from the circumference of the cutter 14 c.

As shown in FIG. 24, the low profile cutting system 10 c may be used ina manner similar to the previously described cutting systems. Forexample, the cutting system 10 c may be used in conjunction with a drillguide 200 b and a cutter drive (not shown) to excise an implant siteconsistent with the previously described methodologies. While theillustrated drill guide 200 b is adapted for accessing and driving thecutter 14 c through the tibia, the low profile cutting system may alsobe used in conjunction with other drill guides and drive systems,including the drill guide 200 illustrated in FIGS. 5 and 6, as well asdrill guides and drive systems having various other configurations.

While the foregoing description of the embodiments of cutting systems,drill guides, and associated methods have been described with referenceto excising a portion of an articular surface of a tibial condyle, thedisclosed systems and methods may suitable be applied to various otherarticular surfaces. The specific embodiments should, therefore only beconstrued as illustrative but not as limiting.

With reference to FIGS. 12 through 14, an implant 500 is shown that maybe used to replace a portion of an articular surface. According to oneembodiment flowing from the foregoing description, the implant 500 maybe used to replace a portion of a tibial condyle in the context of auni-compartmental joint replacement. The disclosed implant 500, however,may also suitably be employed for replacing a portion of an articularsurface associated with other joints and/or other articular surfacereplacement procedures. Accordingly, the description herein should notbe construed as limiting on the application of the implant 500.

As shown, the implant 500 may be configured as a generally cylindricalmember capable of being received in an implant site, e.g. implant site400, defined by an excised portion of bone disposed on an articularsurface, for example created using previously described cutting system10, drill guide 200, and associated methods. The top surface 512 of theimplant 500 may be contoured to generally correspond to a geometry of anarticular surface being replaced by the implant 500. Additionally, oralternatively, the top surface 512 of the implant 500 may have a contourgenerally corresponding to, or complimenting, a geometry of acooperating articular surface or of an implant provided to replace atleast a portion of a cooperating articular surface. As used in anyembodiment herein, a contour generally corresponding to a portion of anarticular surface being replaced may mean that the contour may providesimilar mechanical action in relation to a cooperating articularsurfaces, soft tissue, etc during articulation of the joint. Similarly,a contour generally corresponding to a cooperating implant or articularsurface shall mean a contour providing smooth cooperating action withrespect to such cooperating articular surfaces and/or implants. In anyof the foregoing embodiments, the contour of the top surface 512 of theimplant 500 may be provided based on quantitative and/or qualitativereference to none, any, all, or any combination of the portion of thearticular surface being replaced by the implant 500, the articularsurface receiving the implant, the geometry of a cooperating implant,and/or the geometry of a cooperating articular surface.

According to an embodiment, the top peripheral edge 514 of the implant500 may be relieved. For example, the peripheral edge 514 may berelieved by providing a chamfer or round transition between the topsurface 512 and an upper portion of the sidewall 516 of the implant 500.According to one aspect, relieving the peripheral edge 514 in such amanner may reduce the occurrence of a hard edge that may scrape acooperating implant or articular surface. Relieving the top peripheraledge 514 may accommodate manufacturing or installation tolerances. In asituation in which an edge formed by a projection of the top surface 512and the upper portion of the sidewall 516 would be slightly recessed orstand slightly proud relative to a surrounding articular surface, therelieved peripheral edge 514 of the implant 500 may still allow smoothmovement of a cooperating implant or articular surface across theinterface between the surrounding articular surface and the implant 500,notwithstanding such a slight misalignment. As illustrated in FIG. 14the relieved peripheral edge 514 may produce an indentation or recessedwitness 526 around the perimeter of the implant 500 at the interfacebetween the top surface 512 of the implant 500 and the surroundingarticular surface 528.

With further reference also to FIG. 13, the bottom surface 518 of theimplant 500 may include features to facilitate adhesion between theimplant 500 and bone defining an implant site 400, and/or retention ofthe implant 500 in the implant site 400. In the illustrated embodiment,the bottom surface 518 is shown including a plurality of concentricgrooves, e.g., 520. In a situation in which the implant 500 is to beretained in the implant site 400 using bone cement, the grooves 520 mayfacilitate adhesion between implant 500 and the implant site byincreasing the surface area for adhesive contact. Additionally, oralternatively, the grooves 520 may facilitate retention of the implant500 in the implant site 400 by providing a mechanical or hydraulic lockbetween the implant 500 and the bone cement. Further to this aspect, thegrooves 520 may be provided at an angle to the axis of the implant 500to enhance the mechanical lock against removal of the implant 500 alongthe axis thereof. Similarly, at least some of the grooves 520 may beprovided having an undercut portion along at least a portion of theextent thereof. According to still a further aspect, the grooves 520 mayallow the implant 500 to be bedded down into a layer of bone cementprovided between a bottom of the implant site 400 and the implant 500.Additionally, the grooves 520 may provide a volume for accommodatingexcess bone cement that may be displaced when the implant is bedded downinto a layer bone cement provided between the bottom of the implant site400 and the implant 500. Various other features may suitably be employedto facilitate and/or enhance adhesion between the implant 500 and animplant site 400, and/or retention of the implant 500 in the implantsite 400. For example, the bottom surface 518 of the implant 500 mayinclude a plurality of openings. Similarly, the bottom surface 518 ofthe implant 500 may include a plurality of intersecting grooves, forexample configured in a grid pattern. Various additional and/oralternative features may also be used to facilitate and/or promoteadhesion and/or retention of the implant 500.

In a similar manner as the top surface 512, the implant 500 may includea relieved edge 522 between the bottom surface 518 and a lower portionof the side wall 516. As described with respect to the relievedtransition of the upper peripheral edge 514, the relieved edge 522 maybe provided, for example having, as a chamfer or a rounded over region.The relieved edge 522 may facilitate installation of the implant 500 byallowing a slight initial misalignment between the implant 500 and animplant site 400 into which the implant 500 is being installed. As theimplant 500 is installed into the implant site 400 the relieved edge 522may bear against the articular surface 528 defining the perimeter of theimplant site 400 and allow the implant 500 to slide on the relieved edge522 into proper alignment with the implant site 400. As the implant 500is further inserted into the implant site 400 the conformance betweenthe cylindrical sidewall 516 and the corresponding circularcross-section implant site 400 may draw the implant 500 into properalignment with respect to the implant site 400. Furthermore, therelieved edge 522 may provide a volume for accommodating excess bonecement displaced from a central region of the implant site 400 if theimplant 500 is bedded down into a layer of bone cement provided betweenthe bottom of the implant site 400 and the implant 500. The relievededge 522 may also allow the implant 500 to be fully seated in an implantsite that may not have a sharp corner between the sidewall and bottom ofthe implant site.

The side wall 516 of the implant 500 may include one or morecircumferential grooves 524. The circumferential groove 524 may providean additional site for enhanced adhesion and/or mechanical lock betweenthe implant 500 and a sidewall of an implant site 400. For example, bonecement may be applied in the groove 520 and/or to the sidewall of theimplant site 400. When the implant 500 is installed in the implant site400, bone cement may accumulate in the groove 520 and in contact withthe side wall of the implant site 400. When the bone cement solidifies aprotrusion of bone cement may be formed extending between the side wallof the implant site 400 and the circumferential groove 520 that mayresist extraction of the implant 500 from the implant site 400.According to another aspect, the groove 520 may facilitate handling ofthe implant 500, especially as the implant 500 is inserted through anincision provided for installation of the implant 500, and/or as theimplant 500 is installed into the implant site 400. For example, a toolmay be provided having features sized to be received in the groove 520and having a handle to allow manipulation by a clinician. Accordingly,it may be possible to manipulate the implant 500 by tilting the implant500 as well as translating the implant 500 transversely and axially.

The implant 500 may be provided having features capable of promoting theingrowth of bone to promote retention of the implant 500 in the implantsite 400. For example, the grooves 500 on the bottom surface 518 and/orthe circumferential grooves 524 may allow and/or promote the ingrowth ofbone. Additional and/or alternative features not illustrated herein mayalso be provided to promote and/or allow the ingrowth of bone to promoteretention of the implant 500 in the implant site 400.

An implant 500 consistent with the present disclosure may be formed froma variety of materials selected for various mechanical, physical, andbiological properties. For example, the implant material may be selectedto provide at least some degree of shock absorption or cushioningeffect. Suitable materials may include various biocompatible polymericmaterials, for example, high density polyethylene, ultrahigh molecularweight polyethylene, polyurethane, polyhydroxy-ethyl methacrylate gel,silicone, polyvinyl alcohol gel, etc. Ceramic material such as aluminaor zirconia based materials, as well as various metallic materials, suchas stainless steel, titanium, cobalt-chromium alloys, etc, may be usedto provide an inherent lubrication or low friction surface.Additionally, the implant 500 may include materials that release orproduce therapeutic or lubricating products and may even includebiological materials. Numerous other materials provided the foregoingcharacteristics, and/or other desired characteristics, may be used toproduce an implant 500 consistent with the present disclosure.

As alluded to above, the implant 500 may be used in connection with animplant site 400 created using a cutting system 10, drill guide 200 andthe associated methods for excising an articular surface to create animplant site 400. As also alluded to, the implant 500 may be installedin the implant site 400 by orienting the implant 500 above the implantsite 400 and inserting the implant 500 into the implant site 400 in agenerally axial manner.

According to one particular embodiment, the support member 12 may beused to install the implant 500. The implant 500 may be received in theopening 20 of the support 18. The implant 500 may be retained in theopening 20 by a radial pressure applied by the shoe 24, or by the innershaft received in the lumen of the shaft 16. As described with referenceto the cutting operation, installation of the implant 500 may include atleast partially opening the joint to facilitate access to the implantsite 400 created in an articular surface. The implant 500 may then bedelivered to the implant site 400 using the support member 12. Theimplant 500, retained in the cutter support 18, may be inserted betweenthe cooperating articular surfaces of the femur 100 and tibia 102 andbrought into general alignment with the implant site 400. When theimplant 500 has been delivered to a desired location, the implant 500may be released from the support member 12, for example, by withdrawingthe translating shaft disposed in the lumen of the shaft 16, and therebyreleasing the radial pressure on the implant 500.

Once the implant 500 has been delivered to the implant site, the implant500 may be pressed into the implant site 400, for example, in part usingthe circumferential groove 524 for gripping and/or manipulating theimplant 500. Bone cement may be applied to the implant 500 and/or theimplant site 400. As with the excising operation, installing the implant500 may be accomplished without damaging or removing the meniscus byretracting the meniscus to one side to expose and provide access to theimplant site 400.

The present disclosure also provides a system for replacing a portion ofan articular surface extending around a portion of a curvature of thearticular surface with an implant. The articular surface replacementsystem according to this aspect of the disclosure may allow a greaterarea of an articular surface to be replaced and therein providing areplacement articular surface about a greater range of motion of ajoint. Consistent with one embodiment, the system according to thisaspect of the disclosure may be used to replace a portion of anarticular surface of a femur in a uni-compartmental joint replacement.Consistent with the present system, a relatively large portion of anarticular surface may be replaced by an articular surface implant whileremoving a relatively small mass of articular cartilage, subchondralbone, etc. By removing a relatively small mass of articular cartilageand subchondral bone, the joint may be susceptible to later, moreaggressive replacement therapies. Furthermore, a portion of thearticular surface may be replaced with a minimum of collateral damage,e.g., to surrounding tissue, ligaments, etc.

The system herein may generally include forming an implant site on theportion of the articular surface to receive the implant. The implantsite may generally be formed by a plurality of overlapping excisionsites. The system may generally include establishing an axis for eachexcision site. According to one embodiment, all of the axes mayintersect at a common point. The plurality of excision sites may then beformed by excising a portion of the articular surface and subchondralbone extending inwardly from the articular surface along each respectiveaxis. As mentioned above, each of the excision sites may at leastpartially overlap with an adjacent excision site. The plurality ofexcision sites may, therefore, provide a generally continuous implantsite on the articular surface. An articular surface implant may beinstalled into the implant site.

Consistent with one embodiment, the system herein may be used to replacea portion of an articular surface of a femur, as during auni-compartmental knee joint replacement. With reference to FIG. 15, auni-compartmental joint replacement may include replacing an expanse ofthe articular surface 612 of the femur 613 extending around a portion ofa curvature of a femoral condyle with an articular surface implant 700.Replacing the portion of the articular surface 612 may generally includeproviding an implant site sized to at least partially receive theimplant 700. As stated above, the implant 700 may be received in animplant site including a plurality of at least partially overlappingexcision sites. The excision sites may be formed by excising a portionof the articular surface 612 and a portion of the underlying subchondralbone.

With reference to FIGS. 16 and 17, a plurality of working axes may beestablished extending through the articular surface 612. At least oneaxis may be provided for each excision site to be created to provide theimplant site. With particular reference to FIG. 17, a drill guide 600may be employed to create the plurality of axes having a desiredpredetermined relationship to one another. The axes may be createdextending arcuately around at least a portion of the articular surface612 of the femur 613.

Referring to FIG. 16, an embodiment of a drill guide 600 that may beused to establish a plurality of axes on an articular surface 612 isshown. The drill guide 600 may generally include a body portion 602configured to be disposed on an articular surface 612 on which theimplant site is to be provided. The drill guide 600 may also include afirst and a second drill bushing, generally 604 and 606. The drillbushings 604, 606 may be provided as openings extending throughrespective bosses 608, 610 extending from the drill guide body 602. Thedrill bushings 604, 606 may provide the desired alignment andorientation for holes to be drilled into an articular surface as part ofthe preparation of an implant site.

With further reference to FIG. 17, the drill guide 600 is shownpositioned on an articular surface 612 for use in preparing an implantsite in the articular surface 612. As shown, the body portion 602 may bean arcuate member that may be shaped to generally conform to thearticular surface 612. However, it is not necessary for the body 602 ofthe drill guide 600 conform to the articular surface 612. For example,the drill guide 600 may only make contact with the articular surface 612at the drill bushings 604, 606. Alternatively, the drill guide 600 mayinclude contact features spaced from the drill bushings 604, 606 forcontacting the articular surface 612 and orienting the drill bushings604, 606 in a desired relationship relative to the articular surface612. Variations and modifications in between these two examples maysuitably be employed consistent with the present disclosure.

According to one embodiment, a reference axis extending through thearticular surface 612 may first be established. According to oneembodiment, the reference axis may be provided extending generallynormal to the articular surface 612. The reference axis may beestablished using a normal axis drill guide including a shaft and anaiming feature for projecting a reference axis and a plurality ofarticular surface contacting features for orienting the axis relative tothe articular surface. Such a device is known in the art, for examplefrom U.S. Pat. No. 6,610,067. Other devices may also suitably be usedfor establishing a reference axis.

In one embodiment, the reference axis may be established in the regionof a defect 618 in the articular surface 612, although the referenceaxis may suitably be provided in locations on the articular surface 612.A guide pin may be inserted into the articular surface 612 along thereference axis, for example by providing a relatively small diameterhole in the articular surface 612 and inserting the guide pin into thehole. A larger hole may then be formed extending along the referenceaxis. According to one suitable method, the guide pin may be received ina lumen of a cored, or cannulated, drill bit. The cored drill bit may beused to form a pilot hole centered on the guide pin and extending intothe articular surface 612.

The drill guide 600, described with reference to FIGS. 16 and 17 maythen be used to establish additional working axes through the articularsurface 612. Consistent with an embodiment herein, the additional axesestablished using the drill guide 600 may all intersect at a commonpoint. However, according to alternative embodiments, the several axesestablished through the articular surface may be oriented parallel toone another, or may be otherwise non-intersecting. Furthermore, two ormore of the several axes may intersect at varying points.

According to one embodiment, using the drill guide 600 to establishadditional working axes may include installing a location element in thehole produced along the reference axis. The location element mayinclude, for example, a self tapping screw that may be installed in thearticular surface 612 or subchondral bone therebeneath. The height ofthe location element may be adjusted to a predetermined height relativeto the articular surface 612. According to one embodiment, adjusting theheight of the location element may include mating a positioning insertwith the location element and adjusting the location element, as byadvancing or retracting the location element within the hole, toposition the positioning insert at a predetermined height relative tothe articular surface 612, such as tangential with an arc defined by thearticular surface 612.

The drill guide 600 may be coordinated with the location elementinstalled through the articular surface 612 so that the drill bushing606 may be oriented coaxial with the reference axis defined by thelocation element. For example, a guide rod may be fitted extending fromthe location element, and the guide rod may be received through a drillbushing, e.g. 606, of the drill guide 600. According to such anembodiment, the guide rod and the location element may be providedhaving mating features, such as mating precision tapers. The guide rodmay, therefore, be aligned along the reference axis. Alternatively, thedrill guide 600 and the location element may include cooperatingfeatures allowing the drill guide 600 and location element to becoordinated, e.g. aligned, positioned, etc., in a predetermined manner.The boss 610 extending from the drill guide body 602 may bear against,or otherwise interact with, the location element to position the drillguide 600 at a predetermined height relative to the articular surface612, based on the height of the location element relative to thearticular surface 612.

According to a related alternative embodiment, the drill guide 600 maybe indexed, or positioned, on the articular surface 612 without the useof a location element. Consistent with one such embodiment, a workingaxis relative to the articular surface 612 may be established may beestablished, for example as described above. Also as above, a hole maybe drilled into the articular surface 612 generally along the workingaxis. The boss 610 projecting from the body 602 of the drill guide 600may be at least partially received in the hole drilled into thearticular surface 612. The respective sizes of the hole and the boss 610may be coordinated to achieve a predetermined tolerance and control theamount of movement, or slop, of the drill guide 600 relative to thearticular surface 612. In one embodiment, a snug fit may be achievedbetween the boss 610 and the hole, thereby restricting movement of thedrill guide relative to the articular surface 612.

With the drill guide 600 located relative to the reference axisextending through the articular surface 612, a working axis, oradditional axes 614, 616, may be established relative to the articularsurface 612. The working axis, or working axes 614, 616, may beestablished by drilling holes into the articular surface 612 guided bythe drill bushing 614. Accordingly, the position and orientation of theworking axes 614, 616 may be generally based on reference axis. Whilethe illustrated drill guide 600 only provides two drill bushing 614,616, additional drill bushings may be provided to allow implant sites ofvarious sized to be formed. Furthermore, consistent with someembodiments, the reference axis used for positioning the drill guide 600relative to the articular surface 612 may also provide a working axis,e.g., working axis 616.

According to one embodiment, a location element may be installed intoeach of the holes provided using the drill guide 600. The locationelements installed through the articular surface may, therefore, providea reference, and/or define, the working axes 614, 616 established usingthe drill guide 600. As used herein, it is not necessary for a locationelement to extend along the working axis 614, 616 in order for thelocation element to define the axis. Rather it is only necessary thatthe location element, or a feature thereof, be oriented in apredetermined relationship to the working axis. Depending upon theinitial size of the holes drilled using the drill guide 600, installinga location element may include enlarging the holes to a predeterminedsize. Enlarging the holes may, in turn, include, for example, installinga guide pin into the holes and drilling over the guide pin using a coredor cannulated drill bit. Other techniques may also suitably be employedconsistent with this aspect.

The location elements may include screws, such as self tapping screws.The location elements may be installed having a predetermined depthrelative to the articular surface 612. For example, similar to thepreviously discussed technique, a positioning insert may be associatedwith the location elements, and the height of the height of the elementsmay be adjusted to place the positioning insert at a predeterminedheight relative to the articular surface 612. According to oneembodiment, the location elements may include features, such as taperedopenings, etc., that may allow cooperating elements and instruments tobe placed in a desired position and/or alignment relative to thelocation element and/or the working axis 614, 616 defined thereby.

Installing location elements in each of the holes provided using thedrill guide 600 may be accomplished by articulating the joint tosequentially provide axial access to each of the respective sites.Accordingly, it may not be necessary to separate the joint in order toinstall an implant on the articular surface 612 of one of thearticulating elements. For example, a first site, e.g. a hole in thearticular surface 612 drilled along a first axis 616 using the drillguide 600, may be axially access, for example through an incision, byarticulating the joint so that the desired site is accessible throughthe incision. With the first site accessible through the incision, alocation element may be installed in the hole. The joint may then bearticulated to bring a second site, e.g. a hole in the articular surface612 drilled along the second axis 614, in to a position whereby thesecond site may be accessed through the incision. Accordingly, with thesecond site accessible through the incision a locating element may beinstalled in the second hole. The process of articulating the joint topermit access to different sites on the articular surface may allow theprocedure to be carried out in a minimally invasive manner. That is,rather than providing an incision sufficient to access the entirearticular surface at one time, the size of the incision may be reduced.Furthermore, by accessing various portions of the articular surface 612by positioning the joint in various articulated arrangements, theprocedure may be carried out without completely separating the joint.Therefore, the amount of damage to surrounding tissue may be reduced,however, these aspects are not essential within the context of thepresent disclosure.

Once the working axes 614, 616 have been established, portions of thearticular surface 612 and underlying subchondral bone may be excised toprovide excision sites. For a first excision site, e.g., along theworking axis 616, the joint may be articulated to position the site topermit access as through an incision. With the articular surface 612positioned to provide access to the site, a portion of the articularsurface may be excised. According to one embodiment, the articularsurface may be excised using a drill, rotating cutter, or otherinstrument for excising a generally circular region of the articularsurface and/or subchondral bone. Consistent with an embodiment employinga location element associated with each excision site, the locationelement may include a feature to facilitate locating the excision siteand/or controlling the depth of the excision site.

According to a specific embodiment, the location element may include aprecision tapered socket. A guide rod having a mating precision tapermay be installed into the precision tapered socket. A cutting instrumentincluding a cutting blade configured to rotate around the guide rod maybe used to excise a portion of the articular surface 612 and subchondralbone around the working axis 616 to provide a first excision site. Forexample, a cutting blade, or more than one cutting blades, may beprovided extending from a cannulated shaft. The cannulated shaft may besized to be rotatably received over the guide rod. The first excisionsite may be provided by rotatably driving the cannulated shaft, andthereby driving the cutting blade or blades, about the guide rod.Consistent with this embodiment, the cutting blade or blades may travela circular path centered on the working axis 616. As such, the cuttinginstrument may provide a first excision site defined within the circularpath of the cutting instrument and centered on the first working axis616. In one embodiment, the depth of the excision site may be controlledby providing cooperating features on the cutting instrument and thelocation element. For example, the cutting instrument may have a surfacethat may contact a surface on the location element when the cuttingblade, or cutting blades, have achieved a predetermined depth relativeto the location element. According to an alternative embodiment, theguide rod and or the cutting instrument may include indiciarepresentative of a cutting depth. According to such an embodiment, thedepth of the first excision site may be controlled with reference tosuch indicia.

Depending upon the diameter of the cutting path defined by the cuttingblade or blades, at least a portion of the cutting path defined by thesweep of the cutting instrument may extend outside of the width of thecondyle at the excision site. Accordingly, the first excision site maybe provided having a truncated circular, or an elongated shape. That is,the first excision site may have a first dimension that may be generallydefined by the diameter of the cutting path. The first excision site mayalso have a second dimension that is generally defined by the width ofthe condyle within the cutting path at the excised depth.

After the first excision site has been formed, the joint may bearticulated to a position allowing a second site may be accessed.According to one embodiment, the joint may be articulated to a positionallowing the second site to be accessed through the same incision thatwas used to access the first site. Similar to the process for formingthe first excision site, a second excision site may be formed generallycentered on a second working axis 614 by providing a cutting instrumentthat may be located relative to a second location element. For example,the location element may include a precision taper socket and a guiderod having a precision taper at one end may be received in the precisiontaper socket of the location element with the guide rod generallyextending along the second working axis 614. The cutting instrument mayinclude one or more blades rotatably associated with the guide rod, e.g.via a cannulated shaft of the cutting instrument, such as used toprovide the first excision site. The cutting instrument may, therefore,be employed to provide a circular cutting path centered on the secondworking axis 614. Similar to the process for forming the first excisionsite, the cutting instrument may be rotatably driven about the guiderod, thereby defining a second excision zone within the circular cuttingof the cutting instrument. The depth of the second excision site may becontrolled, e.g. by an interaction between the cutting instrument andthe location element, by reference to indicia on the cutting instrumentand/or the guide rod, etc.

Also similar to the first excision site, according to one embodiment, atleast a portion of the cutting path or excision region defined by thesweep of the cutting instrument may extend outside of the width of thecondyle. Accordingly, the second excision site may be provided having atruncated circular or elongated shape having a dimension defined by thediameter of the cutting path and a second dimension defined by the widthof the condyle within the cutting path at the excised depth.

Consistent with the system according to the present disclosure, thecutting paths defining the first and second excision sites mayintersect, or at least partially overlap one another. The intersectionor partial over lapping of the first and second excision sites mayprovide a generally continuous excised region. The continuous excisedregion may at least partially define an implant site in the articularsurface 612. According to one embodiment, the first and second excisionsites may overlap to a degree wherein one dimension of the continuousexcised region may be generally defined by the width of the condyle atthe excised depth.

The general procedure of forming an excision site using a cutter havinga circular cutting path centered about a working axis defined, e.g., bythe drill guide 600, may be carried as many times as necessary to createan implant site having the desired size, shape, and expanse. As with thefirst and second excision sites, each excision site may intersect, or atleast partially overlap adjacent excision sites. The plurality ofexcision sites may, therefore, provide an implant site that includes aplurality of individual excision sites. As with the first and the secondexcision sites, at least a portion of the cutting path defining any ofthe additional excision sites may extend outside of the width of thecondyle, thereby providing truncated circular, or an elongated, excisionsites, in which one dimension of the excision site may be defined by thewidth of the condyle within the cutting path at the excised depth.

Consistent with an alternative embodiment, the plurality ofintersecting, or at least partially overlapping, excision sites may beformed without the use of location features. According to one suchembodiment, a plurality of working axes 614, 616 extending through thearticular surface 612 may be defined, for example using the drill guide600. The plurality of working axes 614, 616 may be defined, e.g., byholes drilled into the articular surface 612 with the aid of the drillguide 600. According to one embodiment, a guide pin, or guide rod, maybe inserted into each of the holes and extending along the respectiveworking axes 614, 616. A cutting instrument, as described with respectto the foregoing embodiments, may be used in combination with the guidepins, or guide rods, associated with each working axis 614, 616 toexcise a portion of the articular surface 612 and underlying subchondralbone. This method may differ generally only by the omission of locationelements. In a similar embodiment, rather than providing guide pins, orguide rods, associated with each of the working axes 614, 616, a cuttinginstrument may be provided including a pin or rod and one or morecutting blades associated therewith. The pin or rod may be sized to bereceived in the holes in the articular surface 612 provided using thedrill guide 600. An excision site may be formed by inserted a distal endof the pin or rod in a hole associated with a working axis 614, or 616and rotating the cutting instrument with in the hole. Consistent withsuch an embodiment, the hole in the articular surface may serve as abushing for the cutting instrument.

An implant may be installed in the implant site at least partiallydefined by the plurality of intersection or at least partiallyoverlapping excision sites. Accordingly, the implant may replace atleast a portion of the articular surface 612. In one embodiment, one ormore of the location elements may be retained in the subchondral boneafter the excision sites have been formed. The location elements may,therefore, be used as fixation elements for retaining the implant inposition in the implant site. For example, in an embodiment in which thelocation elements include precision tapered socket, the implant mayinclude a fixation feature including a precision tapered post that maybe sized to be received in a precision tapered socket of the locationelement. The tapered post fixation feature of the implant may be pressedinto the tapered socket to provide a secure frictional engagementtherebetween. Alternative methods and features may also, oralternatively, be used for securing an implant in an implant site.

Turning to FIGS. 18 through 20, an embodiment of an implant 700 that maybe used to replace a portion of an articular surface is illustrated in avariety of views. The implant 700 may be used to replace a portion of anarticular surface, for example, extending around a portion of thecurvature of a femoral condyle in a knee joint, as in auni-compartmental knee replacement. An implant 700 consistent with thisaspect of the disclosure may also suitably be used for replacingarticular surfaces associated with other joints and/or other articularsurface replacement procedures. Accordingly, the description hereinshould not be construed as limiting the application of the implant 700.

As shown, an embodiment of the implant 700 may generally include a top,or bearing, surface, generally indicated at 702, and a bottom, or bonefacing, surface, generally indicated at 704. The bearing surface 702 maybe a surface that may replace a portion of an articular surfacereceiving the implant 700. As such, the bearing surface 702 maygenerally face a cooperating articular surface and/or an implantdisposed in or a cooperating articular surface. The bone facing surface704 may face the bone underlying the articular surface being replaced bythe implant 700. In some embodiments herein, the bone facing surface 704may be at least partially received in an implant site.

In the context of an implant suitable for replacing a portion of thearticular surface of a femoral condyle in a uni-compartmental kneereplacement, the arcuate expanse of the bearing surface 702 may be suchthat a portion of the bearing surface may be tangential with a firstplane, and another portion of the bearing surface may be tangential witha second plane that is generally perpendicular to the first plane. Thearcuate expanse of the bearing surface 702, however, may be varied tosuit specific applications, and the foregoing description should not beconstrued as limiting on the scope of the implant 700 herein. Similarly,in context of an implant for replacing a portion of an articular surfaceof a femoral condyle, the implant 700 may have a width that maygenerally be defined by the width of the articular surface of thecondyle. Accordingly, an embodiment of the implant 700 may generallyhave an oblong shape from the perspective of the bearing surface 702 inplan view. As such, the implant 700 may include rounded end regions 703,705 generally. The sides extending between the end regions 703, 705 maybe generally linear, or may be arcuate having a radius larger than theradius of the rounded end regions 703, 705.

According to one embodiment herein, at least a portion of the perimeteredge of the bearing surface 702 may be relieved and not present a hardedge. For example, at least a portion of the perimeter edge of thebearing surface 702 may be chamfered or rounded, for example asindicated at 707 in FIG. 20. When the implant 700 is installed in anarticular surface, the relieved configuration of the perimeter edge ofthe bearing surface 702 may provide a slight recess or witness linebetween the bearing surface 702 of the implant 700 and the surroundingarticular surface. The relieved perimeter edge of the bearing surface702 may facilitate smooth movement of a cooperating articular surface,implant, etc. across the transition between the surrounding articularsurface and the implant 700. For example, the relieved perimeter edgemay accommodate a slight difference in height of the implant 700relative to the surrounding articular surface by not providing a hardedge that may scrape, abrade, or wear an interacting surface.

As best seen in the schematic illustrations of FIG. 19, the implant 700may include a lip 708, 710 on one, or both, ends 703, 705 of the implant700. When the implant 700 is installed into an implant site in anarticular surface, the lips 708, 710 may cut into a portion of thearticular surface and/or subchondral bone surrounding the adjacentregions of the implant site. Accordingly, fraying and/or furtherdeterioration of articular cartilage surrounding the implant site may bereduced or eliminated.

The implant 700 may also include one ore more fixation features 706. Thefixation feature 706 may facilitate securing the implant 700 in adesired position in an implant site. Consistent with the illustratedembodiment, the fixation feature 706 may be provided as protrudingfeature. According to one example, the fixation feature 706 may beprovided as a precision taper. The precision taper may be adapted to beat least partially received in a mating precision taper or a cylindricalopening in a fixation element disposed in bone underlying the articularsurface. For example a screw may provide a fixation element that may bedisposed in the bone underlying the articular surface. The screw mayinclude a central bore that is adapted to receive the fixation feature706 of the implant 700.

According to alternative embodiments, the fixation feature 706 of theimplant may include structures such as a barbed protrusion that may bepressed into a hole formed in the bone underlying the articular surface.The fixation feature 706 may be pressed into the hole and the barbedelements of the protrusion may engage the bone defining the wall of thehole. The engagement between the bone and the barbed elements may resistextraction of the implant 700 from the articular surface. In a similarembodiment, the fixation feature 706 may include a post havingprotrusion and/or indentations thereon. The implant 700 may be securedin a desired location by cementing the post into a hole formed in thebone underlying the articular surface. The protrusions and/orindentations may facilitate mechanical lock between bone cement and thefixation feature 706 of the implant 700. Various other structures,protrusions, receptacles, etc., may be used for securing the implant 700in a desired location.

Consistent with an embodiment of the present disclosure, the implant 700may be configured to have a bearing surface 702 that may approximate thegeometry or curvature of the articular surface being replaced by theimplant 700. In one embodiment the geometry of the load bearing surfacemay be based on the actual articular surface being replaced. Forexample, mapping techniques known in the art may be used to measure theactual geometry of the region of the articular surface being replaced.An implant 700 may then be constructed or selected from a set ofimplants having predetermined geometries to at least generally conformto mapped or measured geometry of the articular surface being replaced.Alternatively, an implant 700 for a specific application may befabricated or selected from a set of standard sized/shaped implants toprovide a general approximation of the articular surface being replaced.Selection or fabrication of an implant 700 may rely on various degreesof quantitative reference to the articular surface being replaced,including no quantitative reference to the articular surface.

Consistent with an alternative embodiment, the implant 700 may beconfigured to provide smooth interaction with a cooperating implant on acooperating articular surface. Accordingly, the cooperating implant maynot have a geometry approximating the geometry of the cooperatingarticular surface. In such a situation, rather than providing theimplant 700 having a geometry approximating the articular surface beingreplaced, the implant 700 may have a contour or geometry that isconfigured to provide smooth interaction with the cooperating implant.In one such embodiment, the implant 700 and the cooperating implant maybe provided as a pair and/or be provided having coordinated geometriesselected to provide a desired, e.g., smooth, interaction with oneanother in an application in a particular joint or in general.

The schematic illustrations in FIGS. 18 through 20 show an embodiment ofa design methodology for an implant 700 is schematically illustrated.Consistent with the schematic depictions, the implant 700 may include aplurality of overlapping implant portions 750 and 752 having a truncatedcircular or oblong configuration. Each of the overlapping implantportions 750, 752 making up the implant 700 may generally correspond torespective excision sites making up the implant site. As such, theoverlapping implant portions 750 and 752 may be arranged in an angularlyand/or radially displaced configuration that may generally correspond tothe angular and/or radial displacement of the respective excision sites.Also similarly, the implant portions 750, 752 may each be orientedrelative to a respective axis utilized to form the respective excisionsites, although other configurations are contemplated herein.

It should be noted that the overlapping implant portions 750, 752 neednot exist as separate, discrete components, but may rather provided as asingle unitary implant 700. Consistent with an alternative embodiment,the individual implant portions may be provided as separate componentsthat may be assembled and/or arranged in the implant site to provide agenerally continuous implant. When the bearing surfaces 754, 756 of theimplant portions are faired, or lofted, to provide a smooth continuoussurface, an implant 700 may be provided having a continuous bearingsurface that may generally represent an original articular surface beingreplaced and/or provide smooth interaction about the range of motion ofthe implant 700 with a cooperating articular surface and/or cooperatingimplant.

Consistent with the previously described system for providing an implantsite, each of the implant portions 750, 752 may have a shape, as viewedlooking at the bearing surface 754, 756 of the implant, which maygenerally correspond to a shape of a respective excision site making upthe implant site on the articular surface 612. In one embodiment thegeometry of the load bearing surface 754, 756 of each implant portion750, 752 may be based on the actual articular surface being replaced ateach excision site. For example, mapping techniques known in the art maybe used to measure the geometry of the actual articular surface beingreplaced in the region of the respective excision sites. The implantportions 750, 752 may have predetermined geometries to at leastgenerally conform to a mapped or measured geometry of the articularsurface being replaced in the region of each excision site.Alternatively, the implant portions may have a standard size/shapeproviding a general approximation of the articular surface beingreplaced. The geometry or contour of the implant portions 750, 752 mayrely on various degrees of quantitative reference to the regions of thearticular surface being replaced, including no quantitative reference tothe articular surface.

Consistent with the foregoing, the implant portions 750, 752 may each bea truncated circular or oblong member, when viewed in plan view from thebearing surfaces 754, 756. By truncated circular or oblong it is meantthat the implant portions 750, 752 may be derived, for example, from acircular implant having a diameter greater than the width of a condyleintended to receive the implant 700. The width of the implant, forexample across the medial-lateral plane of the condyle, may be less thanthe length, i.e., along the anterior-posterior plane of the condyle.Accordingly, a circular implant may be truncated in the medial-lateralplane to have a width that is equal to the width of the condyle at theimplant site. Alternatively, one or more of the implant portions 750,752 may include a width, e.g., across the medial-lateral plane condyle,that is greater than the length of the implant portions 750, 752, e.g.,along the anterior-posterior plane of the condyle. This situation mayoccur if the curvature of a condyle in the anterior-posterior plane issuch that, for an implant having a desired thickness, the articularsurface curves away from the excision site in the anterior-posteriorplane in a smaller dimension than the width of the condyle. Accordingly,the implant portions 750, 752 may be truncated in the anterior-posteriorplane. Further embodiments contemplate a truncated circular implantwherein the implant is truncated in a plane oriented at an angle to theanterior-posterior and medial-lateral planes. Additionally, it iscontemplated herein that the truncated implant may be asymmetrical.

In addition to having a shape that may correspond to an excision site onan articular surface, the implant portions 750, 752 may overlap oneanother to the same degree that the respective excision sites overlapone another to form the implant site. Accordingly, the shape of animplant 700 composed of the implant portions 750, 752 may generallyconform to an implant site including a plurality of excision sites.

While the illustrated implant 700 may be conceptually thought of as twoangularly and/or radially displaced, overlapping truncated circularimplant portions 750, 752, an implant accommodating a larger expanse maybe conceptually provided by more than two overlapping truncated circularimplant portions, each arranged angularly and/or radially displaced fromone another. Similarly, various bearing surface contours may bedeveloped, again conceptually, by providing a plurality of truncatedcircular or oblong implants of varying sizes and arranged at varyingangular and/or radial displacements relative to the other truncatedcircular or oblong implant portions and corresponding to respectiveoverlapping excision sites.

The foregoing design methodology may provide an implant 700 that mayinclude an implant portion 750, 752 corresponding to each excision sitein the articular surface formed to provide the implant site.Accordingly, the implant 700 may not only have a bearing surface thatmay approximate the geometry or contour of the portion of the originalarticular surface 612 being replaced, but the implant 700 may alsoinclude a bone facing surface 704 having a geometry that is adapted tobe received in the implant site formed by a plurality of individualexcision sites.

The foregoing disclosure relates to one possible conceptual methodologyfor designing an implant herein. However, various alternative designmethodologies may also suitably be employed for designing an implantconsistent with the present disclosure. Furthermore, the implant shouldnot be considered to be limited by any particular design methodology bywhich such an implant is obtained. Accordingly, the disclosed embodimentof a design methodology for achieving an implant herein should not beconstrued as limiting the actual implant as disclosed.

Consistent with the foregoing design methodology, an implant 700 may beprovided having a bearing surface approximating an articular surface tobe replaced, each of the overlapping implant portions 750, 752 may beprovided having a load bearing surface 754, 756 that may approximate thegeometry or curvature of a region of the articular surface to bereplaced by the respective regions of the implant 700 associated witheach of the overlapping implant portions 750, 752. The geometry orcurvature of the overlapping implant portions 750, 752 may be based onthe articular surface being replaced, e.g. based on data collected usingmeasuring or mapping techniques. Alternatively, the geometry or contourof the overlapping implant portions 750, 752 may be provided based on ageneral approximation of the respective regions of the articular surfacebeing replaced. Herein, approximating the articular surface beingreplaced, and/or providing a bearing surface based on the articularsurface being replaced, may rely on various degrees or quantitativereference to the articular surface being replaced, including noquantitative reference to the articular surface.

An implant 700 consistent with the present disclosure may be formed fromany suitable biocompatible material. For example, an implant 700 may beformed, in whole or in part, from a metal or metal alloy, e.g.Co—Cr—W—Ni, Co—Cr-M, Co—Cr alloys, Co—Cr-Molybdenum alloys, Cr—Ni—Mnalloys, powder metal alloys, stainless steel, titanium and titaniumalloys. Ceramic materials, such as aluminum oxide or zirconium oxide,may also suitably be used to form an implant herein. Polymeric materialsmay also be used to produce implant according to the present disclosure.For example, polyurethanes, polyethylene, ultra-high molecular weightpolyethylene, thermoplastic elastomers, biomaterials such aspolycaprolactone may all be used herein. Additionally, implants hereinmay be produced from diffusion hardened materials, such as Ti-13-13,Zirconium and Niobium. Coatings, or coated materials, may be employed toprovide porous surfaces, e.g., on bone-contacting surfaces, hydrophilicsurfaces, e.g., on load bearing surfaces. Furthermore, the presentdisclosure contemplates the use of composite implants that may includemore than one material and/or may include more than one type ofmaterials, for example, a metal or metal alloy and a polymeric material.Consistent with this last aspect, composite implants are contemplatedherein that may include load bearing and/or bone facing surfaces thatmay include more than one material and/or more than one type ofmaterial.

1. A method of replacing a portion of an articular surface comprising:forming a first excision site relative to a first axis relative to saidarticular surface; forming a second excision site relative to a secondaxis relative to said articular surface, at least a portion of saidsecond excision site intersecting at least a portion of said firstexcision site; and installing an implant at least partially in saidfirst and second excision sites, said implant having a load bearingsurface comprising a contour based on an original contour of saidarticular surface.
 2. A method according to claim 1, wherein saidimplant site is formed in a femoral articular surface.
 3. A methodaccording to claim 1, wherein said first axis is defined by providing aguide pin extending from said articular surface.
 4. A method accordingto claim 3, wherein providing said guide pin extending from saidarticular surface comprises providing a hole extending into saidarticular surface and disposing said guide pin at least partially insaid hole and extending from said articular surface.
 5. A methodaccording to claim 1, wherein forming said first excision site comprisesexcising said articular surface along a circular excision path aroundsaid first axis.
 6. A method according to claim 5, wherein at least aportion of said circular excision path extends radially from said firstaxis beyond said articular surface.
 7. A method according to claim 1,wherein said first axis is oriented normal to said articular surface ata point of intersection with said articular surface.
 8. A methodaccording to claim 1, wherein said second axis is defined by orienting adrill guide relative to said first axis, said drill guide comprising alocating feature corresponding to said second axis.
 9. An implantcomprising: a load bearing surface comprising a first surface portionhaving a geometry based on an intersection of a first portion of anarticular surface and a first circular projection, and a second surfaceportion having a geometry based on an intersection of a second portionof said articular surface and a second circular projection, said firstand second surface portions at least partially intersecting; and a bonefacing surface.
 10. An implant according to claim 9, wherein said firstsurface portion and said second surface portion are integrally formed.11. An implant according to claim 9, wherein said first surface portionand said second surface portion comprise truncated circular projections.12. An implant according to claim 9, wherein said geometry of said firstand second surface portions are based on a measured contour of saidarticular surface.
 13. An implant according to claim 9, wherein saidbone facing surface is capable of being at least partially received inan implant site formed in said articular surface.
 14. An implantaccording to claim 9, wherein said bone facing surface further comprisesa fixation feature.
 15. An implant according to claim 14, wherein saidfixation feature comprises a tapered post.
 16. An implant according toclaim 9, wherein said first surface portion is tangential to a firstplane and said second surface portion is tangential to a second plane,said first plane and said second plane being perpendicular to oneanother.
 17. A method of forming an implant site comprising: providing acutter; disposing said cutter between a first articular surface and asecond articular surface; providing a drive shaft extending from saidcutter; coupling said drive shaft to said cutter; and rotating saidcutter by rotating said drive shaft.
 18. A method according to claim 17,said drive shaft extending through said first articular surface.
 19. Amethod according to claim 18, wherein providing said drive shaftcomprises providing an access passage extending through said firstarticular surface and through bone underlying said articular surface.20. A method according to claim 17, wherein coupling said drive shaft tosaid cutter comprises providing cooperating features on said cutter andsaid drive shaft capable of transmitting torque from said drive shaft tosaid cutter.